The present invention relates to a method of identifying a plurality of transponders through an interrogation process, to an identification system comprising a plurality of transponders and at least one interrogator, to the transponders and to the interrogators themselves.
Radio Frequency Identification (RFID) systems frequently use collision arbitration, also known as anti-collision protocols, so that a plurality of RFID transponders, often referred to as tags, can be present and separately identified by an RFID interrogator (also known as a reader). There are a number of different types of protocol that can be adopted, the two most common of which are tree walking (using binary search or similar techniques) and random transmit and retry generally referred to as Aloha collision arbitration.
Examples of such systems are described by Marsh et al in U.S. Pat. No. 5,995,017, Palmer et al in U.S. Pat. No. 5,530,702 and Reis et al in EP0467036 and Reis et al U.S. Pat. No. 5,640,151, the whole contents of which are incorporated herein by way of reference. In all these known systems, a tag intermittently transmits an identification code or its identity in response to a signal, command or instruction from an Interrogator. The intermittent response is typically at random or pseudo-random intervals. In many embodiments of the subject invention the systems are such that the interrogator is not required to send commands or conduct a two-way dialogue with the tag or tags, however, the subject invention does not excluded such systems and indeed may be used with such systems. When a plurality of tags is present in the energising zone of an interrogator and if all tags transmit their identities in response to a signal from the interrogator, then tag transmissions may collide or clash. By randomly spacing tag transmissions the probability of collisions is reduced. However, the more tags that are present in the interrogator zone, the greater the repeat interval necessary to ensure that all tag identities are received by the interrogator without clashes of tag transmissions occurring. This problem is increased when the tag transmissions become longer, the longer the data packets the greater the likelihood of the tag transmissions clashing.
Tags are frequently required to carry an identification code as well as additional user encoded data. Also, tags and interrogators are frequently used in open systems where the well known method of data layers is used. The tag—interrogator data exchange and data definition layers are thus often separate entities with no shared knowledge. While the length of the tag ID (TID) may always be pre-determined there is no way to determine how much user data is encoded on the tag or is transmitted by the tag. Therefore the tag packet length could be either a variable length data packet or a fixed length packet with unused bits filled with null information, which is wasteful. It should also be noted that a TID is usually but not always defined in such a way to make it uniquely detectable as a TID data message.
One method used to transmit tag ID or data is to break the tag transmission (message) into a number of transmissions (which we call packets) of equal length and to only transmit the number of packets needed to convey the required data. There are two disadvantages to this method. The first is that the interrogator does not know how many packets to expect. The second disadvantage is potentially more serious. If the transmissions from two separate tags clash or overlap, the interrogator may receive a number of packets from a first tag and then when the first tag has sent its data the interrogator may receive one or more packets from a second tag whose transmission may have been slightly weaker and therefore overridden by the first tag transmission. The interrogator has no way to determine whether all the packets originated from a first tag or that they erroneously originated from a first tag followed by a second or even third tag. Interrogators thus normally need to receive a tag transmission multiple times before deciding it is correct, or have knowledge of the data payload; this is often not practical. A packet, as part of a long transponder transmission, may even not be recognised because of RF noise or collisions. This will result in the interrogator believing that it received two or more transmissions from transponders.